1. Field of the Invention
This invention relates to an air-fuel ratio control system for an engine, and more particularly to an air-fuel ratio control system for an engine in which an air-fuel ratio in an air-fuel mixture fed to the engine is detected by an air-fuel ratio sensor and the air-fuel ratio in an air-fuel mixture to be fed to the engine is controlled to converge on a target value on the basis of the detected air-fuel ratio.
2. Description of the Prior Art
In a recent electronic engine control, the air-fuel ratio in an air-fuel mixture to be fed to the engine is controlled in the following manner.
That is, the amount of fuel to be injected from a fuel injector provided in an intake system of an engine is controlled by a controller which comprises a microcomputer. The controller calculates a basic fuel injection amount (a basic value of the amount of fuel to be injected from the injector) on the basis of the amount of intake air detected by an airflow meter and an engine speed detected by an engine speed sensor, and produces various corrections such as a warming-up increase correction, a starting increase correction, an acceleration increase correction, a heavy load increase correction, an intake air temperature correction, an air-fuel ratio feedback correction and the like according to the operating condition of the engine, thereby determining a final fuel injection amount (a final value of the amount of fuel to be injected).
The feedback correction of the air-fuel ratio is performed on the basis of the output of an air-fuel ratio sensor (which may be of an O.sub.2 sensor) which is provided in an exhaust system when the operating condition of the engine as determined, for instance, according to the engine speed and the engine load is in a predetermined feedback zone.
The O.sub.2 sensor outputs a voltage the value of which increases as the air-fuel ratio decreases and greatly changes near the stoichiometric air-fuel ratio. The controller compares the output voltage of the O.sub.2 sensor with a reference voltage which corresponds to the stoichiometric air-fuel ratio, and reduces the amount of fuel to be injected from the injector when the former is higher than the latter and increases the amount of fuel to be injected from the injector when the former is lower than the latter so that the air-fuel ratio approaches the stoichiometric air-fuel ratio.
In some of the conventional electronic control engines, the controller performs the following learning control together with the feedback correction in order to compensate for fluctuation in the basic air-fuel ratio due to variation and/or change with time in properties of various parts such as the engine, the airflow meter, the injector and the like.
While the feedback control is performed, the controller samples the average of the feedback correction values over a predetermined time at regular intervals and stores the up-to-date value of the average in a memory. The value stored in the memory is referred to as "a feedback correction learning value". The controller calculates the amount of fuel to be fed to the engine using also the feedback correction learning value as a parameter. This increases controlling accuracy of the air-fuel ratio. Further, by performing the learning control on the basis of the learning value stored in the memory while the feedback control is not performed, the air-fuel ratio can be caused to approach the stoichiometric air-fuel ratio.
Further, the controller also carries out the following abnormality processing relating to the feedback correction. That is, when the air-fuel ratio control is correctly performed, the output voltage of the O.sub.2 sensor forms a waveform which oscillates up and down about the reference voltage. If the output voltage of the O.sub.2 sensor is fixed to the higher level side or the lower level side for a long time and does not change, it can be considered that the O.sub.2 sensor is cold and is not active yet, or the O.sub.2 sensor itself fails or the air-fuel ratio control system fails. Accordingly, when the output voltage of the O.sub.2 sensor does not change across the reference voltage for a predetermined time (e.g., 10 seconds), the controller determines that the feedback control system including the O.sub.2 sensor is abnormal and interrupts the feedback correction of the air-fuel ratio.
In Japanese Patent Publication No. 58(1983)-24610, it is proposed to set the reference voltage for determining whether the O.sub.2 sensor is active at a value which is different from the conventional reference voltage described above and conforms to the particular engine in order to accurately determine whether the O.sub.2 sensor is active and to quickly start or interrupt the feedback correction.
However, when whether the O.sub.2 sensor is active is determined on the basis of whether the output voltage of the O.sub.2 sensor does not change across a particular reference voltage for a predetermined time, it is difficult to set the reference voltage and/or the predetermined time so that the feedback control of the air-fuel ratio can be performed for a long time with high stability and reliability.
That is, when the reference voltage is set at a low value, minute noise in the output level can exceed the reference voltage even if the O.sub.2 sensor is not active, which can lead to misjudgement and delay in detecting an abnormality. On the other hand, when the reference voltage is set at a high value, it takes a long time to determine that the O.sub.2 sensor becomes active and the feedback control cannot be quickly started.
Further when said predetermined time is short, there arises a problem when the learning control is performed. That is, when the feedback correction learning value is cleared in response to, for instance, disconnection of the battery, the output voltage of the O.sub.2 sensor can be fixed to an upper limit value or a lower limit value until the feedback correction learning value is reset to a reasonable value. Accordingly, when said predetermined time is short, the feedback correction is interrupted in such a case, and the feedback correction learning value cannot be updated, which can lead to a state in which the feedback correction and the learning control cannot be started. On the other hand, when the predetermined time is long, it takes a long time to detect that the O.sub.2 sensor and/or the control system fails.